1. Field of the Invention
This invention relates generally to machining instruments, and in particular to surgical instruments that are capable of machining hard tissues in relatively confined or constrained environments or spaces, and yet remain reliable for extended periods.
2. Description of Related Art
In a variety of surgical procedures there is a need to machine hard tissues such as bone, i.e. cut, abrade, obliterate or remove hard tissue by mechanical means. In some of these procedures, the human anatomy provides little room to maneuver and properly position a machining instrument. Furthermore, in some procedures precise machining must be done at an angle relative to the direction at which the machining instrument is inserted. These difficulties, caused at least in part by the environment in which the machining instrument is used, create significant design constraints. For example, the components used to make the machining instrument must often be small so that the device can fit into and operate in restricted or constrained anatomical spaces. At the same time, the device may require translational gears to provide the necessary angled machining, which take up much needed space in the instrument. The tension between these conflicting requirements often results in significant design tradeoffs for such equipment.
In addition to meeting these design constraints, the devices must be capable of withstanding the forces exerted on it during repeated uses. These forces may be relatively significant, particularly in view of the size limitations of the components.
Furthermore, in typical surgical machining procedures one must be concerned with the efficiency of the machining operation, and seek to avoid the generation of heat caused by inefficient machining. Excessive heat generation will result in unwanted damage to the living hard tissue and other surrounding tissues in the form of thermal necrosis. In particular, when machining bony tissue, excessive heat can kill osteoblasts in the vicinity of the machining operation, which can lengthen healing times and limit desired bony ingrowth into devices implanted into cavities formed by the machining device. To maximize machining efficiency, the rotational speed and torque of the machining element should be optimized. This often requires a machining instrument design that provides low friction rotation of a machining element at a relatively high speed and torque. Since the instrument must be sized to fit within constrained spaces, little room is available inside the instrument for the gearing, bearing, or other drive mechanisms to enable a low friction, high-speed, high-torque design. Such space constraints are often met by the use of high gear ratios, resulting in suboptimal pinion teeth geometry. The resulting wear significantly limits the life span of such surgical instruments.
In addition, these types of machining instruments are repeatedly exposed to harsh environments that can also shorten their useful life. Specifically, these instruments are often subjected to repeated heat cycles and corrosive cleaning agents during sterilization or autoclaving prior to each use. Therefore, the materials used to fabricate the machining instruments must be biocompatible and capable of withstanding the extreme sterilization temperatures that typically exceed 135xc2x0 C. In addition, the repeated thermal expansion and contraction of the materials may result in a degradation of some of the mechanical interfaces in the device. As a result, it is desirable to design the devices to minimize such degradation.
One example of this type of device is a milling tool used for the machining of a vertebral body endplate. A vertebral body endplate might be machined in order to prepare the endplate to receive spinal disc prosthesis. An example of procedures for implanting a spinal disc prosthesis is described in U.S. patent application Ser. No. 09/783,860, filed Feb. 13, 2001, and a Continuation-in-part thereof, filed Aug. 7, 2001, the entire contents of each of which are hereby incorporated by reference. In such a procedure the machining instrument must be small enough to be inserted into the intervertebral disc space, which is relatively small. In addition, the machining surface must be positioned at essentially a 90xc2x0 angle relative to the longitudinal axis of the instrument as it is inserted into the disc space. Consequently, this requires a drive mechanism having relatively small drive components that are capable of milling at approximately 90xc2x0 relative to the direction the device is inserted. This application thus requires sophisticated instrumentation that is small enough to be maneuvered within constrained spaces in the human body, and yet includes a small and robust drive mechanism capable of facilitating machining at difficult angles and capable of withstanding repeated uses.
Examples of an instrument for machining a vertebral body endplate are described in U.S. Pat. No. 6,083,228. The ""228 Patent disclosures does not provide any details on how the device disclosed therein is constructed, and does not address the issues outlined above.
A particular instrument suitable for machining vertebral endplates has been designed and manufactured by Spinal Dynamics Corporation. This design is described in general in U.S. patent application Ser. No. 08/944,234, filed Oct. 6, 1997, and Ser. No. 09/783,860, filed Feb. 13, 2001, and a Continuation-in-part thereof, filed Aug. 7, 2001, the entire contents of each of which are hereby incorporated by reference. The Spinal Dynamics design is shown in FIG. 1 and includes a cutting element 2, a gear 4, and a bearing assembly 6 that are all mounted in a housing 8. In accordance with this design, adhesives are used to secure bearing assembly 6 in housing 8. In addition, gear 4 includes an axial hub 12 that is press fit to an outer gear ring 10. Although this design is effective to machine a vertebral body endplate, the inventors of this application have discovered that, over time, the usefulness of the device may become less reliable. In particular, the repeated use of the device may result in failure of the outer gear ring 10 as a result of the stresses exerted by the press fit of axial hub 12 and/or loads applied during use. In addition, the repeated sterilization of the device may compromise the effectiveness of the adhesives used to secure bearing assembly 6 to the housing. While these instruments are certainly sufficient to achieve a successfull intervertebral implantation, there remains a need for improved instruments that are more durable and can withstand repeated uses.
The invention relates to an apparatus for machining hard tissue, such as bone, as well as softer tissue associated therewith. The apparatus provides high speed rotation, high torque, and low friction, and is adapted to fit into and operate within small, constrained spaces within anatomical structures of humans or other animals. The apparatus allows for machining tissue from areas and at angles that are difficult for the operator to reach otherwise. The apparatus is robust, and contains components that are capable of withstanding repeated exposure to extreme temperatures as the apparatus is reused, and autoclaved or otherwise heat sterilized prior to each use.
As explained in more detail below, the apparatus takes power supplied by a drive shaft and transfers it approximately 90xc2x0, allowing the operator to mill tissue approximately perpendicular to the path of entry of the apparatus into the tissue. This makes the apparatus very suitable for removing tissue from joints. As an example, the apparatus can be used very effectively to remove tissue from vertebral joints, including cortical bone. This might be done in preparing the intervertebral space to receive an implant or prosthesis, for example.
In the apparatus of the invention, power is taken from a rotating shaft, e.g., a geared shaft, having an axis essentially parallel with the longitudinal axis of the apparatus and with the path of entry into the anatomical structure to be machined. The rotating shaft meshes with gear teeth on a perpendicularly oriented gear disposed within a housing on one end of the apparatus. The rotation of this gear also causes the rotation of a gear hub attached to the gear, and which is attached to a moveable member of a bearing assembly. The moveable member of the bearing assembly can move relative to a non-moveable member of the bearing assembly that is affixed to the housing, and is desirably separated from the non-moveable member by one or more friction reducing members. The cutting element of the instrument is attached to the gear or the gear hub, whose rotation causes the cutting element to also rotate. The turning blades of the cutting element can then be brought into contact with the tissue to be removed.
In a particular embodiment, the cutting element can be a cutting disk having axially extending blades or flutes on one side thereof, and an axially extending shaft on the other side, which extends into an axial opening in the gear or gear hub or both, and tightly fits therein.
The gear hub and bearing assembly are desirably press fit together, and the gear and gear hub are desirably fit together by interlocking complementary noncircular geometries, and all three elements are locked together to form a drive assembly which is attached to the housing of the apparatus and to which the cutting element can be removably attached. This locking function may be performed by any suitable mechanism. Desirably, the elements of the drive assembly are locked together with two locking members, one of which is disposed adjacent the gear and the other disposed adjacent the gear hub. These locking members are adapted to generate opposing forces that hold the drive mechanism together, e.g., by generating compressive forces.
The housing of the apparatus contains an opening adapted to receive the drive assembly. This opening contains several different segments, which are generally coaxial, and as explained below, have different diameters to accommodate different portions of the drive assembly.
The invention can be more clearly understood by reference to the attached drawings, the brief description thereof below, and the detailed description of specific embodiments of the invention, all of which are illustrative of, and not limiting of, the invention recited in the appended claims.